In recent years, as digital cameras have become technologically advanced and widespread, digital cameras are used for various shooting purposes, and a number of new picture representation methods that were missing in film-camera days are arising. Included among the methods is a method of producing a new picture utilizing a large number (generally over a few hundred) of still images captured by performing interval shooting with a digital camera secured on a tripod. More specifically, lighten compositing, which is a type of compositing processes, is adopted on some occasions to produce a photo in which the number of trails of fireflies moving inside a picture display is increased or a photo in which trails of stars moving across a night sky due to diurnal motion of the earth are captured.
A lighten compositing process is performed according to the following procedure, for example. First, a first image, which is an image captured first, of a large number of still images to be processed is written to a compositing memory area as an initial value. Luminance comparison is made between a pixel of a second still image, which is an image captured next to the first image, and the same pixel of the first still image written to the compositing memory area, and a brighter one of the pixels is rewritten to the compositing memory area. This operation is performed on every pixel, thereby causing a composite image of the first still image and the second still image to be stored in the compositing memory area. Thereafter, as for each of third and following still images, the operation of making luminance comparison between a pixel, of a still image to be processed and the same pixel of a composite image in the compositing memory area and rewriting a brighter one of the pixels to the compositing memory area is performed on every pixel, and this step is repeatedly performed on every still image to be processed.
A composite image that looks as if the image was captured by long exposure is obtained by perforating such a lighten compositing process as described above using a large number of still images captured by interval shooting with a digital camera. The lighten compositing process is generally executed by software installed in a personal computer (hereinafter, referred to as a “PC”). More specifically, a photographer generally captures a large number of images by performing interval shooting using a digital camera, transfers the images to the PC, and performs the lighten compositing process using the software on the PC.
Described below are advantages and disadvantages of each of approaches that are long exposure with a conventional film camera, normal long exposure with a digital camera, long exposure by stacking with a digital camera, and a lighten compositing process using a PC. The long exposure by stacking with a digital camera is, for example, long exposure that utilizes the technique described in patent document 1.
(1) Long Exposure with Film Camera
<Advantage>                When long exposure is performed on a low-intensity subject, a formula that doubling exposure duration makes a resultant photo brighter by a degree corresponding to 1 EV does not hold, and a film is less sensitive to light (that is, a film generally has a characteristic called “reciprocity law failure”). Accordingly, long exposure on the order of hours in a dark environment can be performed utilizing this characteristic (however, it should be noted that limitation to 1 to 2 hours is imposed in Japan where night skies are bright, although the limitation depends on an environment, when an ISO 100 film is used). This allows, by shooting a night sky, obtaining a photo in which trails of stars am captured without making a low-intensity portion, which corresponds to background of the stars, too bright.        
<Disadvantages>                Experience is required to determine exposure because of reciprocity law failure.        This approach is prone to color cast.        In an environment where subjects that differ largely in luminance are mixed, blown out highlights are undesirably produced in a brighter portion of a photo (therefore, an urban scene with stars trailing as long as 1 hour cannot be captured with this approach).        The longer the light trails, the greater an f-stop number or the lower a sensitivity of a film to be used becomes. Therefore, trails of dark stars (faint stars) are not captured (this is due to a principle generally called as limiting magnitude).        A user cannot view a composite image in progress.        A user cannot view a resultant image immediately after shooting.        
(2) Normal Long Exposure with Digital Camera
<Advantages>                Because digital cameras are insusceptible to reciprocity law failure even with a low-intensity subject, doubling exposure duration makes a resultant photo brighter by a degree corresponding to 1 EV.        This approach is suitable for a purpose of shooting photos in which stars are still on a condition that a wide-angle lens is used, exposure time is set to equal to or shorter than 20 seconds, and ISO sensitivity is set to a high sensitivity.        Color correction can be performed easily.        A user can view a resultant image immediately after shooting.        
<Disadvantages>                Because digital cameras are insusceptible to reciprocity law failure, this approach is unsuitable at all for shooting on the order of hours (it is substantially impossible to perform such shooting with this approach because an entire photo becomes blown out highlights).        Random thermal noise, although it depends on a performance of an imaging device and a shooting environment, appears on a display when the digital camera does not include a cooling mechanism. This can be avoided by: cooling; however, in that case, the digital camera also needs to include a mechanism for preventing condensation resulting from cooling, and tends to be large equipment.        It is difficult to reduce random noise with this approach.        A user cannot view a work in progress of exposure.        
(3) Long Exposure by Stacking Using Digital Camera
<Advantages>                With this approach, reading from imaging devices and digitization are performed at fixed intervals. Accordingly, shades of gray can be kept even when long exposure by stacking is performed for a period longer than that of normal long exposure of the digital camera.        Color correction can be performed easily.        A user can view a work in progress of exposure.        A user can view a resultant image immediately after shooting.        
<Disadvantages>                Because digital cameras are insusceptible to reciprocity law failure, this approach is unsuitable at all for shooting on the order of hours (it is substantially impossible to perform such shooting with this approach because an entire photo becomes blown out highlights).        It is difficult to reduce random noise with this approach.        Continuity of time is broken because overall exposure time is interrupted by time, although being short, necessary to read data from the sensors (this does not matter at a focal length on a wide-angle side in star tail shooting; however, the longer the focal length is, the more greatly this matters).        
(4) Lighten Compositing Processing Using PC
<Advantages>                Exposure can be determined easily because exposure that is determined for shooting of one image is nearly continuously used with this compositing method.        Color correction can be performed easily.        Even in an environment where subjects that differ largely in luminance are mixed, this approach is less prone to blown out highlights (in a case of shooting an urban scene with star trails, duration of the trails can be lengthened to desired time not later than morning unless a happening such as incoming of light that is unpredictable at start of shooting does not occur).        High-sensitivity shooting can be performed because exposure can be determined for shooting of one photo. Even trails of dark stars (faint stars) that cannot be captured with a film can be captured.        Thermal noise is negligible because exposure is performed not so much as to cause thermal noise to affect image quality. Therefore, the digital camera needs not to include a special cooling mechanism.        The greater the number of images to be stacked, the greater random noise can be reduced in a degree incomparable to the other approaches.        This approach allows producing a composite image in which fireflies look as if the are increased in number by intentionally breaking continuity of time. Continuity of time is intentionally broken also in star trail shooting on some occasions.        
<Disadvantages>                Continuity of time is broken because overall exposure time is interrupted by time, although being short, necessary to read data from the sensors (this does not matter at a focal length on a wide-angle side in star tail shooting; however, the longer the focal length is, the more greatly this matters).        A user cannot view a composite image in progress because the lighten compositing process is performed on a PC after shooting.        A user cannot view a resultant image immediately after shooting because the lighten compositing process is performed on a PC after shooting.        
As described above, conventional long, exposure with a digital camera ((2) and (3) described above) is far inferior to long exposure with a film camera ((1) described above). A main reason therefor is that digital cameras are insusceptible to reciprocal law failure. Furthermore, long exposure with a film camera also gives many inconveniences.
In contrast, the lighten compositing process using a PC ((4) described above) is utilized as an approach that has not only a number of advantages of the other approaches ((1) to (3) described above) but also greatly excels the other approaches. However, this approach requires that a large number of images obtained by interval shooting with a digital camera are transferred to the PC to undergo the compositing process in the PC. Therefore, a photographer can neither envision a finished image during shooting nor can view a resultant photo at a photo shooting location immediately after completion of shooting.